The variability in the duration and form of the canid play bow was studied in infant coyotes, wolves, wolf-dog hybrids, beagles, and adult free-ranging dogs. Both duration and form showed marked stereotypy. It appears that the role of this context-specific social signal in the communication of play intention has been fostered by selection for "morphological" stereotypy.
Bekoff, M. (1977). Social communication in canids: Evidence for the evolution of a stereotyped mammalian display. Science, 197(4308), 1097-1099.